Acoustic device



Jam 29, 1929. 1,700,457

A. l.. 'rHuRAs ACOUSTIC DEVI GE Filed Aug. 2, 1926 hasta@ a. 29,1929." y

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LABORATORIES, INCORPORATED, or

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S i P A NEW JERSEY, ASSlGNOR TQ BELL TELEPHONE i NEW' YORK`, N. vY.,...i. CORPORATION OF NEW i,1oo,457

EN olrlca.

ACOUSTIC EEVICE.

Application filed August 2, 1929,k Serial No. 126,523.

riFhis invention relatesto acoustic devices and particularly to attenuators for acoustic apparatus.

The principal object of the invention is to attenuate sound energy without introducing appreciable distortion. A more specific object is to control the sound outputi of phonograph rreproducers,

` loud speakers and other acoustic devices.

-stant impedance over the sound frequencyV range and a substantially uniform frequency loss characteristic when employing` impedanceelements having reactance components rather than being pure resistances, it is preferable to employ elements consisting of substantially pure resistance. f y In accordance with a preferred form ofthe invention the resistancel elements consist of gauze discs yplaced on each side ofthe sound generating device or diaphragm and shutters are employed to regulate the effective areas of the gauze discs to vary the attenuation.

The invention is particularly applicable to acoustic devices employing a diaphragm contained in a partly enclosed case, yone side of Vwhich is connected to a horn or other soundradiating device. Sound volumelregula-tors for such devices have-been devised in the past, as disclosed, for example,` in Patent No. 1,229.7 80 granted to W. Opel, June 12, 1917.

' This patent shows a volume regulator-,com-

prising two superposed perforated discs one on each side of a layer of felt inserted in the `diaphragm case between the4 diaphragm and the sound transmitting passage, these perforated discs being rotatable to vary the volume of the radiated sound. However, as far as applicant is aware there have been ngo volume regulators having variable' impedance elements rnd which give'the same impedance for `different attenuation settings and have unimpedance.

lform attenuation characteristics over a fairly large frequency range. v

rlfhelnvention may be better understood by referrlng to the accompanying drawing in which: Fig. 1 shows the attenuator of this invention-applied to a phonograph re ro- 6o ducer F1g. 2 is a detail sectional view o the device shown in Fig. l taken along the line 2 2; Fig. 3 asectional view taken along the line 3 3 of Fig. 2 and having portionsbroken away to better show the invention; andl Figs. A

4 to 7 inclusive are diagrammatic representations of thevimpedance relations which exist in the ldevice.

Referring to the drawing, the phonograph reproducer 8 employing the attenuator of this 70 invention is connectedv to lthe-tone arm v9 which leads to a horn (not shown). The peripheral portion of the conical metallic y diaphragm 10is clamped, by means of a threaded ring 35 to thering 11 which is se- 75 ,cured to the case 12 by the screws 13. The diaphragm is driven atits apex. by the stylus arm 14 to the other end of 4which the needle 15 is securedA by the set screw 16.*" Slight-ly spaced from one side of the diaphragm is a thin metallic gauze member 17 having extremely ine perforations, a material employing 250, 0.004 parallel/wires to the vinch in one direction and 28, 0.009f. wires to the inch extending intersectingly to the 0.004 wires having been found/ satisfactory. ,Such a member has a substantially pure resistance y This gauze member is rigidly secured to the ring 29 which is joined to the ring 30 by metallic spokes 40-40, as illus- 90 .trated 1n F ig. 2, the ring 30 in turn being threaded l into the ring 1.1. lVith this arrangement the air space between the diaphragm 10 and the gauze member-17 com`- municates directly with the outside air lthrough the opening between the rings 29 and 30 when the adjacent shutter 18 is en- This shutter is also used tov tirely open. cover a portion lof the gauze member 17 to vary its resistance, the creased asa greater portion of the gauze is covered. ,Onv the other side of the diaphragm the tone arm 9 is fitted into the metallic tube case 12 bymeans v of the metallic spokes 20. Slightly spaced 105 19 which is connected to the from this side of the diaphragm and about the opening through the tube 19 is positioned airing-shaped ygauze'mernber 21 which vis lof resistance being insimilar material to that employed in the gauze member 17. "A shutter 22 is positioned adjacent to the gauze member 21 to vary its impedance. ated simultaneously` by movement of the thumb screw 23 transmitted through the gears 24 and 25 respectively. These gears are connected to the common shaft 26 and mesh with the toothed wheels 27 and 28 respectively. The relative movement of the shutters -1s proportional to the diameters of the gears 24 and 25. 'llhe operation of the shutters is so adjusted that the shutter on one side of the diaphragm'is closed when the one on the opposite side is open, the relative rate of opening and closing of the two shutters being such thatthe impedance into which the diaphragm works remains substantially constant over the iound frequency range for` all values of attenuation.` When the shutter 18fis set for maximum opening and shutter 22 is closed, the minimum attenuation or ymaximum sound radiation takes lplace and, conversely, when the shutter 18 is entirely closed and the shutter 22 is setforI its maximum opening, the device gives a. maximum attenuation. The vshutters 18 and`22 may be of any type, the ones shown in the drawing being of the type commonly employed in cameras as the iris diaphragm. Fach shutter is made up of a number of curved sectors which are pivoted at one end and moved simultaneously at the other end so as to enlarge or decrease the size ef the opening at the center of the shutter. These sectors are pivoted at onesen'd to the .ring 11 by means of the pins 31 and are Figs. 4 to 7 inclusive are diagrammatic repi resentations of the mechanical impedance relations which exist in the sound reproducer. Figure 4 shows these relations for the condition in which the shutter 22 on the horn side of the diaphragm is entirely closed and the `into the horn at the throat end, which is a substantially pure resistance, is represented by 11144. K ratio of displacement toapplied force. The

'llhe shutters 18 and l22 are oper- Compliance may be defined as the' modas? mitting path.- For this condition the shutter,

22 is partly opened `and the shutterv 18 is partly closed. 1n this figure M5, represents the mass of the diaphragm, C52 its compliance and R57 the impedance looking into the horn, at the throat end. ,l have in addition, for this setting of the attenuator, a series arm coin-l prising the compliance CM of the air space in back of the diaphragm and the impedance Z55 of the gauze member 17 in parallel with each other and a shunt arm comprising the compliance C53 of the airspace in front of the diaphragm and the impedance ZSG of the gauze member 21 in shunt with one another.

The compliances C53 and C54 are made of such a size that rthey may be neglected in making design computations covering a large portion of the frequency range employed in speech and music. However, at some ofthe higher frequencies of the sound frequency range, the effect of their presence may become noticeable.

Neglecting the effect of these compliances the impedance diagram is shown in Fig. 6 in `which Z62 and Zn, represent the impedance of the gauze members 17 and 21respectively, and RC4 the throat impedance of the horn. ln operating the shutters the attenuation is increased by closing the shutter 18 and opening the shutter 22, which operation is equivalent to increasing the impedance Z62 and decreasing Zm. In order to maintain the inipedance Z'G5 looking away from the diaphragm equal to Re, for all values of attenuation, it is essential that Figure is the` same as Figure 6 except that the impedances ZG2 and ZG3 are shown as resistances R7L and R73 respectively. 5 ln View of the relative difficulty of correctly proportioning reactive impedance elements in the attenuator of this invention, it is preferable to employ impedance elements having a negligible reactance component thus apf proaching the condition shown inA Fig. 7 lt has been found `tl1at the impedance of thin gauze members having very fine perforations is practically a pure resistance. Under these conditions in order to maintain the input impedance 7T1 of the attenuator equal to the horn impedance R74 it is necessary to mainous elements,y

' of various forms may be employed,

R an las) llt is seen that the acoustic attenuation networks of the general form s'hown in Fig. 7 are substantially distortionless since the network impedanceis maintained substantially constant for all values of attenuation and for a given setting, the loss introduced by the attenuator isnsubstantially uniform over a large portion of the sound frequency range.

lit is Vto be understood that the use of the acoustic attenuator of this invention is not limited to a phonograph reproducer'but may be readilyndapted to other acoustic devices, as, ,for instance, the electromagnetic telephone receiver. While the use of metallic gauze impedance elements having a very` small cross-section and extremely fine perforations is desirable, many other materials as for instance, a transverse section of bamboo or a layer of cellotem What is claimed is:

l. An acoustic attenuator comprising impedance elements in shunt and series relation phragm for attenuatin generated by said diap" I phragm, an attenuator comprising an imi -phragm,

'.impedance elements, and means for varying vterminal impedance of said acoustic attenu-` to the line of propagation of wave energy, and means for simultaneously varying the impedance of said elements to vary the attenua'- tion produced thereby. y y

2. Anl acoustic attenuator comprising impedance elements in shunt and series relation to the line" of propagation of wave energy,

and means for varying the impedance of said l elements to vary the attenuation produced thereby While maintaining the impedance of said attenuator substantially constant.

3.1n an acoustic deviceincluding a diaphragm, an acoustic attenuator comprising means positioned on -both sides of said diathe sound energy ragm substantially. uniformly over a large portion of the sound frequency range. K

4. In an acoustic device including a diaelement on each ,sideof said diaand means for varying the magiii-4 tude of said impedance elements. v

5. In an acoustic device includinga diapedancev Jphragm, an attenuator comprising impedance elements of substantially puresresistance on each side of said diaphragm, and means for varying the magnitude of said impedance elements.

6. A variable acoustic attenuat'or compris# ing an impedancenetwork having varlable tain thetinput impedance of said network sub'- stantially constant 'over a wide frequency and attenuation range.

'E'. In an acoustic device having a diaphragm and a sound transmission path arrangedto receive sound waves from one side of said diaphragm, means for attenuating the sound Waves transmitted from said diaphragm to said path, said means comprisingl a perforated -member leading into the free.

air from the side ofsaid diaphragm associated with saidpath, aysecond perforated member leading into the free airfrom the l other side of said diaphragm, and means for simultaneously varying the eective area ofboth of said members, to vary the attenuation introduced by saidattenuating means.

8. An acoustic device according to the preceding claim characterized by means for simultaneously varying the eective area of the perforated members while maintaining the acoustic impedance of the attenuating means substantially constant,` 4

9. In an acoustic device i'having" a diaphragm anda sound transmission path arranged to receive sound waves from one side soy of said diaphragm, means for attenuating the sound Waves transmitted froml said diaphragm to said path, said means comprising a thin gauze member leading into the free air from the sid-e of said diaphragm associated with said'path, a second thin gauze member leading into of said diaphragm, and means for simulthe free air from the other side taneously varying the effective area of both of said members to -vary the attenuation intro-` duced by said attenuating means.

10. In an acoustic device having a diaphragm and a sound transmissionk path arranged to receive sound waves from one side of said diaphragm, means for attenuating the sound waves transmitted from said f diaphragm to said path, said means comprising a thin, finely perforated gauze member and leading into the free air from the side of, said diaphragm associated with said path, a second thin, finely perforated gauze member Vof said diaphragm, and adjustable shutters adjacent toeach of said gauze members for simultaneously varying the effective area of both of said members to vary the attenuation introduced by said attenuating means.

11. In anpacoustic device including a dial phragm, a variable acoustic attenuator cmprismg impedance elements of substantially leading into the free air from the other side.`

llO

pure resistance onl each side of said diay a portion of said vgauze members,l

the impedance of said elements so as to mainator remainspsubstantially constant over a large portion of the sounol frequency range While the attenuation of said Variablc'acoustic attenuator is varied.,v

12. ln a mechanical phonograph inclulingI 5 a sound reproduccr and a sound radiator, a

variable acoustic attenuator comprising a Variable sound mpeding element in series with the impedance of said sound radiator anol a. second Variable sound impcding elemenlf in l0 parallel with the impedance of said soundl momo?" radialorh means for simultaneously varying the impedance of saicl sound impending clements for controlling the volume' of sound radiated from said mechanical phonograph Without introducing appreciable distortion.

ln witness whereof7 ll hereunto subscribe my name this 31st day of July A. D., 1926.

ALBERT L rHUiaAs, 

